Method of forming a protective coating on molybdenum



3,019,516 Patented Feb. 6, 1962 3,019,516 METHOD OF FORMING A PROTECTIVECOATING N MOLYBDENUM James C. Holzwarth, Birmingham, James R. Hornaday,In, Royal Oak, and Charles W. Vigor, East Detroit, Mich, assignors toGeneral Motors (Iorporation, Detroit, Mich, a corporation of Delaware INo Drawing. Filed May 13, 1950, Ser. No. 813,034

12 Claims. (Cl. 29-198) This invention relates to a protective coatingfor molybdenum and molybdenum base alloys. It pertains particularly to aprocess for providing a molybdenum base metal turbine blade or similararticle with a composite coating which possesses good high temperatureoxidation resistance and excellent ductility with respect to impactindentation and creep deformation.

The nickel base alloy and cobalt base alloy blades commonly used todayin gas turbine engines normally have maximum service temperatures ofapproximately 1800 F. to 1900 F. This limitation necessarily restrictsthe performance and efficiency of these engines. Molybdenum has asatisfactory high melting temperature and suflicient potentialavailability to warrant investigation as a high temperature turbineblade material. However, it possesses poor oxidation resistance attemperatures of 1200 F. or above. Therefore, molybdenum isunsatisfactory for use in turbine blades which necessarily are exposedto extremely hot oxidizing gases. During recent years attempts have beenmade to correct this deficiency by adding small amounts of variousalloying elements to molybdenum. However, such attempts have beenunsuccessful since the resultant products still do not possess adequateoxidation resistance at 2000 F. and above, the temperature conditionsunder consideration in the present invention.

' Likewise, considerable effort has been expended in the development ofcoatings for molybdenum in order to permit this metal to be used atelevated temperatures under oxidizing conditions. In the past, forexample, nickelchromium alloys have been applied to molybdenum sheets byroll cladding, and an alloy layer of this type protects molybdenum fromoxidizing environments at elevated temperatures. Prior to the presentinvention, however, such processes for coating molybdenum usuallyinvolved heating the base metal above its recrystallization temperature,thereby producing embrittlement. Heretofore molybdenum also has beencoated with chromium-nickelboron and chromium-nickel-silicon-boronalloys. However, the utility of these coatings is limited by theirrelatively low melting points and the tendency for the nickel to diffuseinto and embrittle the molybdenum.

A principal object of the present invention, therefore, is to provide aprotective coating for molybdenum and molybdenum base alloys winch isoxidation-resistant in air at temperatures as high as 2200 F. Inaddition, if gas turbine blades or similar articles formed of molybdenumbase alloys are to be coated, the coating must possess sufiicientductility to resist mechanical impact, creep deformation and thermalshock. A further object of the invention, therefore, is to provide aprocess for producing such a coated article which possesses acombination of these desirable properties to an outstanding degree.

These and other objects are attained with this invention by successivelycoating molybdenum or molybdenum base alloys with layers of chromium,nickel-chromium alloy and nickel-chromium-boron alloy. The presentprocess produces a satisfactory protective coating on finish machinedmolybdenum articles of complex shape without heating the base metalabove its recrystallization temperature. Of course, it will beunderstood that the term molybdenum, as used herein, is intended to alsoencompass molybdenum base alloys which require protection againstoxidation at elevated temperatures.

More specifically, the coating employed in accordance with thisinvention is a composite which includes an electroplated chromium layerand an overlay of an alloy comprising about 15 to 25% chromium and thebalance substantially all nickel. However, alloys of this type having achromium content as high as 50% also are useful for this purpose. Annickel-20% chromium alloy is generally preferred. The chromiumelectroplate functions as a barrier to prevent the nickel from diffusinginto the molybdenum since such diffusion produces rapid and seriousembrittlement of the base metal. The chromium preferably is depositedfrom a bath under operating conditions which produce a soft, crack-freechromium deposit.

A dry hydrogen annealing treatment is employed after the chromiumelectrolplating process to further soften the plate and establish adifiusion bond to the molybdenum. This heat treatment also serves as aninspection procedure for determining the degree of adherence of thechromium plate to the molybdenum. A one-hour treatment at 2000 F.appears to be optimum, although the temperature and time may varyappreciably depending on the particular molybdenum base metal beingtreated. 1700 F. is considered the minimum diffusion temperature, butannealing at that temperature requires too long a time to becommerically practical. The maximum permissible temperature is thatimmediately below the recrystallization point of the'molybdenum ormolybdenum base alloy article being treated. Hence, the maximumtemperature mayvary from 1800 F. to 3000 F., but the recrystallizationtemperatures of the typical molybdenum base alloys under considerationrange from 2200 F. to 2600 R, depending on the alloy composition and theprior working history of the article. At the practical maximum heattreating temperature of 2200 F., the heat treating time is reduced toapproximately one-half hour.

It is recognized that the chromium electroplate is, in itself, a goodoxidation-resistant coating for molybdenum. As is well known, however,chromium becomes embrittled as a result of high temperature exposure inair, principally because of nitrogen pickup, and hence a protectiveoverlay is required for high temperature applications in oxidizing orair atmospheres.

We have obtained excellent results with a chromium plate having athickness of about 1 mil, although the chromium generally functions as asatisfactory barrier layer when its thickness ranges from about 0.5 milto 4 mils. However, normally it is not particularly advantageous to usea chromium plate thicker than 2 mils. When a relatively thin chromiumlayer is applied, it is particularly important to insure that this layeris soft and crack-free.

The nickel-chromium layer may be built up over the chromium plate byflame spraying a'powdered pro-alloy of about 80% nickel and 20%chromium, for example. A highly satisfactory layer also is formed byflame spraying to a controlled weight per unit area a powdered metalmixture of an alloy of 80% nickel and 20% chromium to which has beenadded a nickel-chromium-boron alloy in an amount equal to approximately5% to 15% of the nickel-chromium alloy. The ternary alloy facilitatessintering in a subsequent heat treatment by melting and bonding thenickel-chromium alloy particles together, thereby permitting the use ofa lower sintering temperature. While it eliminates tendencies towardshrink cracking, it does not reduce the porosity of the nickel-chromiumalloy.

The nickel-chromium-boron alloy preferably comprises about to 17%chromium, 2% to 4% boron and the balance substantially all nickel.However, satisfactory results can be obtained with a chromium content ashigh as 20% and a boron range of approximately 0.5% to 6%. Of course,small amounts of the other minor constituents may be present in thealloy without detracting from its utility. For example, up to about 5%silicon and 1% carbon further lower the melting point of thenickelchromium-boron alloy, but the use of a large amount of silicon isundesirable because silicon, as compared with boron and carbon, will notdiffuse out of the alloy to as great a degree during heat treatment.Likewise, a high carbon content should be avoided since it tends to formbrittle carbides in the nickel-chromium alloy. Iron also may be in thealloy, and 5% or so iron is frequently found in alloys of this type.Other elements, such as copper, cobalt, tungsten and molybdenum, may bepresent in small quantities in the sprayed coating layer withoutadversely affecting it. An example of a suitable commercially availablenickel-chromium-boron alloy is one consisting of approximately 13.5%chromium, 3.5% boron, 4.5% silicon, 4.5% iron, 0.8% carbon and thebalance nickel.

We have found that generally the nickel-chromium alloy should be appliedin an amount to form a layer weighing about 0.1 to 0.15 gram per squarecentimeter, 0.125 gram per square centimeter being considered optimum atpresent. In the as-sprayed or sintered condition a nickelchromium alloylayer of this weight will have a thickness of approximately 5 to mils,depending on the density of the sprayed mass. A metal powder of -60 to+200 mesh size is preferred.

After the nickel-chromium layer has been formed on the surface of thechromium electroplate, the coated article is subjected to a second dryhydrogen heat treatment cycle which reduces any metal oxides generatedand entrapped by the flame spraying. This procedure produces an overlayof a clean but fairly porous metal network somewhat resembling apowdered metallurgy sintered compact. The heat treatment sinters thenickelchromium particles, but the sprayed layer remains essentiallyporous. The high temperatures (i.e., approximately 2400 F.) necessaryfor complete fusion of the particles are to be avoided since exposure tosuch temperatures recrystallizes the base molybdenum. Of course, thetemperature should be high enough to reduce any metal oxides present andto sinter the nickel-chromium layer to a sufiicient extent to preventchipping of the layer during handling. Likewise, the coating must notbecome so 'hot as to close the pores and shrink and craze the coating. Atemperature range similar to the one applicable in the initial annealingtreatment is generally appropriate. However, it is advantageous to beginthis second heat treatment at a relatively low temperature, such as 1700F., to permit presintering and eliminate excessive shrinkage andcracking of the nickelchromium alloy coating. 7

The porous outer layer thus formed is subsequently impregnated with anickel-chromium boron alloy of the aforementioned type. This may beaccomplished by flame spraying a powdered alloy over the sinterednickel-chromium alloy in an amount equal to about 0.1 to 0.15 gram persquare centimeter. In general, the weights per unit area of thenickel-chromium alloy and the nickel-chromium-boron alloy are preferablyabout equal. In the case of the latter, the thickness of the sprayedoverlay will be about 3 to 10 mils when this amount of metal powder isused. A nickel-chromiumboron alloy having a particle size of about to+200 mesh produces excellent results.

The coated article is thereafter subjected to a'brief. high temperatureheat treatment in a dry hydrogen atmosphere to cause thenickel-chromium-boron alloy to 'melt and penetrate into the pores "ofthe matrix nickelchromium alloy. It is desirable to heat the coatedarticle as quickly as possible to a temperature of 2200 F. to 2250" F.and to maintain this temperature until the pores of the nickel-chromiumalloy are filled. This normally requires ten to twenty minutes. Therapid high temperature heating is advantageous since it causes theternary alloy to infiltrate the nickel-chromium alloy matrix before theboron and silicon, if any is present, are completely diffused out of theoverlay. Such premature total diffusion would raise the melting point ofthe nickel-chromiummoron alloy excessively before the pores of thematrix were filled. Above 2250 F. the alloy becomes too fluid and tendsto erode away and drip off the molybdenum base metal article beingcoated. The minimum heat treating temperature is the flow point of thenickel-chromium-boron alloy (i.e., approximately 1800 F.), but the alloywill not flow satisfactorily in the short time desired unless thetemperature is about 2100 F. or above.

A programmed diffusion treatment cycle is next employed to reduce thelocal boron concentration gradients in the coating by solid statediffusion into the nickelchromium alloy. A substantially dry hydrogenatmosphere is again employed with annealing temperatures of 2000 F. to2250 F. being appropriate. While this diffusion treatment temperaturemay be appreciably lower than the temperature initially used to melt thenickel-chromium-boron alloy overlay, temperatures as low as 1800 F.require heat treatment times which are too long to be practical. As aresult of this heat treatment sequence, the outer layer ofnickel-chromium alloy becomes homogeneous and has a melting pointconsiderably above the heat treatment temperature. The total compositeprotective layer, including the chromium plate, preferably has athickness of about 7 or 8 mils, although this layer may vary from 5 to20 mils and still be effective.

While the'use of a dry hydrogen atmosphere in the coating eifectivelyprotects molybdenum for more than 1,000 hours at a temperature of 2000F. in air and for approximately 800 hours at a temperature of 2200 F.Moreover, the ductility of the above-described coating on molybdenum ishighly satisfactory with respect to ballistic impact resistance andcreep deformation. Our tests show that coated molybdenum stress-rupturespecimens can undergo appreciable elongation without cracking under hightemperature creep loads in air. For example, the coating has withstoodat least 5% creep elongation at 1800 F. and 10% creep elongation at2000" F. in'air. When the impact resistance of the coating was evaluatedby a ballistic impact test, the coating was shown to be capable ofindentation without spalling or cracking at temperatures of 1800 F. .to2000 F. Thermal shock is equal'to'the best of any other fused metalcoating, cladding or electrodeposited coating.

Attention should be given to the geometry of th article to be coatedand, in general, it should be free of sharp edges with fillet radiibeing as large as practicable.

Of course, all' surfaces must be accessible for metal' spraying anduniform electroplating. With these generalities in mind, the followingspecific example illustrates a preferred embodiment of the coatingprocedure hereinbeforedescribed. In general, we have found itdesirableto initially grit blast the surfaces of the mqlyb- Following thissurface preparation of the molybdenum, chromium is electroplated on thearticle to a depth of about 1 to 1.5 mils. Before the actualelectrodeposition step, the article is advantageously subjected to anabrasive scrub followed by a water rinse, an acid etch and a secondwater rinse. A 50% hydrochloric acid solution may be effectively used asthe etchant.

A useful chromium plating bath is one containing approximately 33 ouncesper gallon of chromic acid, 0.2 ounce per gallon of sulphuric acid and1.57 ounces per gallon of an additive designed to promote the formationof a soft and crack-free plate. Additive CPA 1800, currently sold by theDiamond Alkali Company of Cleveland, Ohio, is an example of acommercially available product of this type. A bath temperature of about170 F. has proved to be satisfactory. The article to be electroplated isclamped onto an electrode and immersed in the bath with the article asthe anode. A 3-volt applied potential can be used and the currentpolarity reversed and adjusted to about 0.75 ampere to 1.5 amperes persquare inch for a period of one-half to one hour. Next, the voltage isreduced to approximately 2 volts, and the article is removed and rotatedto expose the previous contact points. The molybdenum article is thenreplaced in the bath at the 2-volt level, the voltage being increased tothe plating level and the plating continued from one-half to one hour.This latter sequence of steps is repeated until the desired platingthickness is produced, after which the plated article is rinsed in waterand dried. In general, we have found that four contact rotations aredesirable if the molybdenum base metal article is to be completelycoated.

The chromium plated article is then annealed for one hour at atemperature of 2000 F. in a substantially dry, flowing hydrogenatmosphere. In order to obtain the proper dryness of the hydrogen, tankhydrogen can be passed through a catalytic reactor to combine with anyimpurity oxygen present to form water. The gas is then dried by passingit through a cooling coil chilled by a solid carbon dioxide-acetonemixture. The dew point of this gas is approximately 90 to 100 F.

Following this annealing treatment for the chromium plate, the powderedmixture of 80% nickel and 20% chromium, to which has been added byweight of the aforementioned nicikel-chromiumboron powdered alloy, isflame sprayed to uniformly cover the electroplate. Ne have obtainedexcellent results with a powder of approximately l50 mesh. Our workindicates that 0.12 to 0.13 gram of nickel-chromium alloy overlay persquare centimeter of the original surface area produces highly desirableresults. After this flame spraying step, the as-sprayed nickel-chromiumalloy coating is annealed in dry hydrogen for two hours at a temperatureof about 1700 F. and for an additional hour at 2000 F. The coatedarticle is then cooled to room temperature under the hydrogenatmosphere.

Next, a layer of the above-described powdered chromium-nickel-boronalloy is fiarne sprayed to uniformly cover the surface of the platedarticle. In general, we have found that the preferred weight range isapproxi mately 0.12 to 0.13 gram per square centimeter of sprayedsurface. The article is then fusion heat treated for about minutes at atemperature of approximately 2200 F. in a dry hydrogen atmosphere. It isdesirable to charge the cold sample in the hot furnace while under thisatmosphere. When the coated molybdenum article is still in the furnace,it is advantageous to subject it to a diffusion annealing treatment bylowering the furnace temperature to 2000 F. for one hour, raising it to2l00 F. for one hour, followed by a one-hour anneal at 2200 F. Such aprocedure paces the diffusion rate of boron into the matrix 80% nickel-%chromium alloy.

The advantages of the coating described herein are apparent whencompared with the coatings heretofore used for the same purpose. Forexample, silicide coatings,

which are extremely resistant to oxidation at very high temperatures,are so brittle that they cannot withstand impact blows or anysignificant amount of stress applied to the underlying molybdenum.Aluminunrchromiumsilicon coatings likewise exhibit good oxidationresistance and have excellent thermal shock resistance, but suchcoatings fail to protect the molybdenum when impacted. The lack ofductility of these two types of coatings results from the fact that theprimary oxidation resistance is provided by brittle inter-metalliccompounds.

Various other coatings for molybdenum have been tried in the past. Suchtreatments include the simple application of nickel-chromium-boron alloycoatings, electrodeposited nickel and nickel-chromium alloy combinationsand claddings. While coatings of this type are very ductile, theygenerally fail to protect molybdenum at temperatures above 2000 F.during extended exposure in air. Moreover, the application of suchcoatings recrystallizes the underlying molybdenum, :and the coatingshave only fair thermal shock resistance. On the other hand, theprotective coating which is produced by the procedure embodying thepresent invention can be applied without recrystallizing the base metaland without sacrificing ductility. Moreover, the thermal shockresistance is improved and the effective maximum exposure temperature isincreased approximately 200 F.

While our invention has been described by means of certain specificexamples, it is to be understood that the scope of the invention is notto be limited thereby except as defined in the following claims.

We claim:

1. A process for forming a coated molybdenum base metal article which isresistant to oxidizing gases at temperatures above 2000 F., said processcomprising coating surfaces of said article with a thin, crack-freelayer of chromium, flame spraying a thin porous coating of an alloycomprising about 50% to nickel "and 15 to 50% chromium over saidchromium layer, thereafter flame spraying a thin layer of an alloycomprising about 74% to 89.5% nickel, 10% to 20% chromium and 0.5% to 6%boron over said coating, and diffusing said nickelchromium-boron alloyinto said nickel-chromium alloy.

2. A method of protecting a molybdenum base metal article againstoxidation in air at elevated temperatures by providing said article witha ductile oxidation-resistant surface coating, said method comprisingapplying a thin layer of relatively soft, crack-free chromium on saidarticle, heating said article to further soften said chromium layer andestablish a diffusion bond to the molybdenum base metal, flame sprayinga thin porous coating of nickel-chromium alloy comprising about 50% to85% nickel and 15% to 50% chromium over said chromium layer,subsequently reducing any metal oxides generated and entrapped duringsaid flame spraying, flame spraying a thin layer ofnickel-chromium-boronalloy comprising about 74% to 89.5% nickel, 10% to20% chromium and 0.5% to 6% boron over said porous coating, thereafterheating said article to melt said nickel-chromium boron alloy and causeit to impregnate said porous coating, and continuing to heat saidarticle to reduce local boron concentration gradients in the coating bysolid state diffusion into said nickel-chromium alloy.

3. A method of protecting a molybdenum base metal article against hightemperature oxidation in air, said method comprising electroplating athin layer of chromium on surfaces of said'article, heating said articlein a noncarburizing, nonoxidizing atmosphere for a period of time and ata temperature sufficient to soften the chromium plate and establish adiffusion bond to the molybdenum base metal, flame spraying a thinporous coating of a metal comprising about 15% to 50% chromium and 50%to 85% nickel over said chromium, subsequently heating said article insaid atmosphere at a temperature and for a period of time sufficient toreduce any metal oxides generated and entrapped during said flamespraying, flame spraying a thin powdered alloy layer comprisingapproximately to 20% chromium, 0.5% to 6% boron and the balanceprincipally nickel over the formed nickel-chromium alloy coating, andheating said article in said atmosphere at a temperature and for aperiod of time sufficient to melt the nickel-chromium boron alloy andcause it to penetrate the pores of the nickel-chromium alloy;

4. A process for forming a protective coating on a molybdenum base metalarticle which comprises electroplating a thin barrier layer of chromiumon surfaces of said article, heating said plated article in asubstantially dry hydrogen atmosphere for a period of time and at atemperature sumcient to soften the chromium plate and establish adiffusion bond to the molybdenum base metal, thereafter flame spraying apowdered metal comprising about to 50% chromium and the balancesubstantially all nickel over said chromium layer in an amount equal toapproximately 0.1 to 0.15 gram per square centimeter, subsequentlyheating said article in said atmosphere at a temperature and for aperiod of time sufficient to sinter the formed nickel-chromium alloyoverlay and reduce any metal oxides generated and entrapped in saidoverlay during said flame spraying, flame spraying a powdered alloycomprising about 10% to chromium, 0.5% to 6% boron and the balanceprincipally nickel over said overlay in an amount equal to about 0.1 to0.15 gram per square centimeter, and thereafter heating said coatedarticle in said atmosphere at a temperature and for a period of timesufficient to melt the nickelchromium-boron alloy and cause it toinfiltrate pores of said sintered nickel-chromium alloy.

5. A process for forming a protective coating on an article formed of abase metal selected from the class consisting of molybdenum andmolybdenum base alloys, said process comprising coating surfaces of saidarticle with a thin, crack-free layer of chromium, applying a thinporous coating of a nickel-chromium alloy comprising about to 85% nickeland 15% to 50% chromium over said chromium layer, applying a thin layerof nickelchromiurn-boron alloy comprising about 74% to 89.5% nickel, 10%to 20% chromium and 0.5% to 6% boron over said coating, thereafterheating said article under nonoxidizing conditions to melt saidnickel-chromiumboron alloy and cause it to penetrate pores of saidnickelchrornium alloy, and continuing to heat said article to reducelocal boron concentration gradients in the coating by diffusion intosaid nickel-chromium alloy.

6. A process for forming a protective coating on an article formed of abase metal selected from the class consisting of molybdenum andmolybdenum base alloys, said process comprising applying to surfaces ofsaid article a relatively soft, crack-free layer of chromium having athickness of about 0.5 mil to 2 mils, annealing said chromium platedarticle at a temperature of approximately 2000 F. to 2200 Fjin asubstantially dry nonocarburizing atmosphere which is reducing withrespect to chromium, forming a porous layer of nickel-chromium alloyover said chromium layer in an amount equal to about 0.1 to 0.15 gramper square centimeter, sintering said porous layer at a temperature ofabout 1700 F. to.2200 F. in said atmos-' phere, thereafter applying aporous coating of an alloy comprising approximately 10% to 20% chromium,0.5%

to 6% boron and the balance principally nickel over said nickel-chromiumalloy layer in an amount equal to about 0.1 to 0.15 gram per squarecentimeter, and heat' treating' said article in said atmosphere at atemperature of ap- 7; A process for forming a ductile; high temperatureoxidation-resistant coating on a molybdenum base metal proximately 1800F. to 2250 Fato melt the nickelchromium-boron alloy coating and cause itto impregnate; said porous nickel-chromium alloy layer; and continuingto diffusion heat said article to reduce local boron concenarticle whichcomprises cleaning surfaces of a molybdenum base metal article,electroplating a relatively soft, crack-free layer of chromium on saidarticle to a depth of about 0.5 mil to 4 mils, flame spraying a porousoverlay of a metal powder on said chromium plate in an amount equal toapproximately 0.1 to 0.15 gram per square centimeter, said powderedmetal comprising about 15 to 50% chromium and the balance substantiallyall nickel, sintering said overlay in a substantially dry hydrogenatmosphere at a temperature of about 1700 F. to 3000 F., flame sprayinga powdered alloy comprising approximately 10% to 20% chromium, 0.5% to6% boron and the balance principally nickel on said overlay in an amountequal to about 0.1 to 0.15 gram per square centimeter, and thereafterheating said article in said atmosphere at a temperature ofapproximately 1800 F. to 2250 F. to melt the nickel-chromiumboron alloyand cause it to penetrate into the pores of said overlay, and continuingto heat said article to reduce local boron concentration gradients inthe resultant coating by solid state diffusion.

8. A process for forming a ductile, high temperature oxidation-resistantcoating on a molybdenum base metal article which compriseselectroplating relatively soft, crackfree chromium on said article to adepth of about 0.5 mil to 2 mils, heating said plated article in asubstantially dry, nonoxidizing, noncarburizing atmosphere to furthersoften the chromium plate and establish a diffusion bond to themolybdenum base metal, thereafter flame spraying over said chromiumplate a thin porous layerof a metal comprising approximately 15% to 50%chromium and the balance substantially all nickel, sintering said layerin said atmosphere at a temperature of about 1700 F. to 3000 F.,thereafter flame spraying a thin coating of powdered alloy comprisingabout 10% to 20% chromium, 0.5% to 6% boron and the balance principallynickel over said layer and heating said article in said atmosphere at atemperature of approximately 1800 F. to 2250 F. to

melt said nickel-chromium-boron alloy and cause it to penetrate intosaid porous layer, and continuing to diffusion heat said article in saidatmosphere to reduce local boron concentration gradients in theresultant coating by diffusion into the nickel-chromium.

9. A process for forming a protective coating on a turbine blade formedof a base metal selected from the class consisting of molybdenum andmolybdenum base alloys,

.said process comprising electroplating relatively soft,

crack-free chromium on said turbine blade to a depth of about 0.5 mil to4 mils, heat treating said chromium plated turbine blade at atemperature of approximately 2000 F.

to 2200 F. in a substantially dry hydrogen atmosphere for a timesufficient to further soften the chromium plate and establish adiifusion bond to the base metal,thereafter flame spraying a porousoverlay of a powdered metal on said chromium platein an amount equal toabout 0.1 to 0.15 gram per square centimeter, said powdered metalcomprising approximately 15% to 25% chromium and to nickel, heattreating said overlay in a substantially dry hydrogen atmosphere at atemperature of about 1700 F. to 2200 F. for a period of time suflicientto reduce any metal oxides generated and entrapped during said flamespraying, thereafter flame spraying a powdered alloy comprisingapproximately 10% to 17% chromium, 2% to 4% boron and the balanceprincipally nickel on said overlayin an amount equal to about 0.1 to0.15 gram per square centimeter, subsequently heat treating said platedturbine blade in a substantially dry hydrogen atmosphere at atemperature of approximately 2000 F. to 2250 :F. to melt thenickel-chromium-boron alloy and cause it to penetrate said porousoverlay, and continuing to heat'said turbine blade insaid atmosphere toreduce localboron-concentration gradients in the resultant coating bysolid state diffusion into the nickel-chromi- 10. A process for forminga protective coating ,on a molybdenum base metal article which comprisescleaning surfaces of a molybdenum base metal article, electroplatingrelatively soft, crack-free chromium on said article to a depth of about1 to 2 mils, annealing said chromium plated article at a temperature ofapproximately 2000 F. to 2200 F. in a substantially dry hydrogenatmosphere for a time suflicient to further soften the chromium plateand establish a diftusion bond to the molybdenum base metal, thereafterflame spraying a porous layer of a metal powder having particle sizes ofapproximately 60 to +200 mesh over said chromium plate in an amountequal to about 0.12 to 0.13 gram per square centimeter, said metalpowder comprising an alloy of approximately 15% to 25% chromium and thebalance substantially all nickel to which has been added anickel-chromium-boron alloy, subsequently sintering said layer in saidatmosphere at a temperature of about 1700 F. to 2200 F. for a period oftime suflicient to reduce any metal oxides generated and entrappedduring said flame spraying, cooling said article to room temperatureunder said atmosphere, thereafter flame spraying a metal powder coatingcomprising approximately to 17% chromium, 2% to 4% boron and the balanceprincipally nickel over said nickel-chromium alloy layer in an amountequal to about 0.12 to 0.13 gram per square centimeter, said powderhaving particle sizes of approximately 60 to +200 mesh, and fusion heattreating said article in said atmosphere at a temperature ofapproximately 2100 F. to 2250 F. to melt the nickel-chromium-boroncoating and cause it to infiltrate the pores of said porous layer, anddifiusion heating said article in said atmosphere at a temperature ofabout 2000 F. to 2250 F. to reduce local boron concentration gradicutsin the resultant coating by solid state diflusion into thenickel-chromium.

11. A molybdenum base metal article having a surface coatingcharacterized by high temperature oxidation re sistance and excellentductility with respect to impact indentation and creep deformation, saidarticle comprising a molybdenum base metal, a barrier layer ofrelatively soft,

crack-free chromium on surfaces of said base metal and bonded thereto,said chromium layer having a thickness of approximately 0.5 mil to 4mils, and a thin coating comprising a matrix of nickel-chromium alloyover said barrier layer and fused thereto, said matrix being impregnatedwith an alloy comprising about 10% to 20% chromium, 0.5% to 6% boron andthe balance substantially all nickel, the local boron concentrationgradients in said coating having been reduced by diffusion into thenickel-chromium alloy.

12. A turbine blade having a surface coating characterized by hightemperature oxidation resistance and excellent ductility with respect toimpact indentation and creep deformation, said turbine blade comprisinga base metal selected from the class consisting of molybdenum andmolybdenum base alloys, a diffusion barrier layer of relatively soft,crack-free chromium electroplated on surfaces of said base metal anddiffusion bonded thereto, said barrier layer having a thickness ofapproximately 0.5 mil to 2 mils, and a thin coating comprising a matrixof a nickelchromium alloy over said barrier layer and fused thereto,said matrix being present in an amount equal to about 0.1 to 0.15 gramper square centimeter and comprising about 15% to 25% chromium and thebalance substantially all nickel, said matrix being infiltrated with analloy comprising approximately 10% to 17% chromium, 2% to 4% boron andthe balance substantially all nickel, the local boron concentrationgradients in said coating having been reduced by solid state diffusioninto the nickel-chromium alloy, the coating on said turbine blade havinga total thickness of about 5 to 20 mils.

Turner et al Sept. 25, 1956 Montgomery et al Dec. 25, 1956

7. A PROCESS FOR FORMING A DUCTILE, HIGH TEMPERATURE OXIDATION-RESISTANTCOATING ON A MOLYBDENUM BASE METAL ARTICLE WHICH COMPRISES CLEANINGSURFACE OF A MOLYBDENUM BASE METAL ARTICLE, ELECTROPLATING A RELATIVELYSOFT, CRACK-FREE LAYER OF CHROMIUM ON SAID ARTICLE TO A DEPT OF ABOUT0.5 MIL TO 4 MILS, FLAME SPRAYING A POROUS OVERLAY OF A METAL POWDER ONSAID CHROMIUM PLATE IN AN AMOUNT EQUAL TO APPROXIMATELY 0.1 TO 0.15 GRAMPER SQUARE CENTIMETER, SAID POWDERED METAL COMPRISING ABOUT 15% TO 50%CHROMIUM AND THE BALANCE SUBSTANTIALLY ALL NICKEL, SINTERING SAIDOVERLAY IN A SUBSTANTIALLY DRY HYDROGEN ATMOSPHERE AT A TEMPERATURE OFABOUT 1700*F. TO 3000*F., FLAME SPRAYING A POWDERED ALLOY COMPRISINGAPPROXIMATELY 10% TO 20% CHROMIUM, 0.5% TO 6% BORON AND THE BALANCEPRINCIPALLY NICKEL ON SAID OVERLAY IN AN AMOUNT EQUAL TO ABOUT 0.1 TO0.15 GRAM PER SQUARE CENTIMETER, AND THEREAFTER HEATING SAID ARTICLE INSAID ATMOSPHERE AT A TEMPERATURE OF APPROXIMATELY 1800* F. TO 2250* F.TO MELT THE NICKEL-CHROMIUM-BORON ALLOY AND CAUSE IT TO PENETRATE INTOTHE PORES OF SAID OVERLAY, AND CONTINUING TO HEAT SAID ARTICLE TO REDUCELOCAL BORON CONCENTRATION GRADIENTS IN THE RESULTANT COATING BY SOLIDSTATE DIFFUSION.